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Wednesday, March 31, 2010

Script aliases in the style of git

The Git version control software lets you run its commands either through as a single program name followed by the command as an argument (such as git status), or as individual scripts (such as git-status).

The advantage of the second form is that you can use the text completion feature of your shell to save you some typing. It's a personal preference...

It is implemented by a series of symlinks from the longer command names to a single git executable. git itself gets the name of the executable it was called as and breaks that down to get the name of the command.

It is easy to create your own version of this. Here it is in Ruby...

The 'primary' script is called myscript.
#!/usr/bin/env ruby
script = File.basename($0)
if script =~ /^\S+?_(.*)$/
command = $1
command = ARGV.shift
puts "command #{command}"

if ARGV.length > 0
puts "args #{ARGV.join(', ')}"

Create an alias to it by adding a suffix (the sub command name), separated by some delimiter and symlinking this to the primary script:
$ ln -s myscript myscript_cmd_0
$ ln -s myscript myscript_cmd_1

The script looks at how it was called ($0 in Ruby) and sees if it can split off a command. If it does then it takes any other arguments as they are presented. If you call the script as the primary script followed by a separate command then it shifts ARGV to get the command. These examples show how it works.
$ ./myscript_cmd_0 foo bar
command cmd_0
args foo, bar
$ ./myscript_cmd_1 foo bar
command cmd_1
args foo, bar
$ ./myscript cmd_1 foo bar
command cmd_1
args foo, bar

You don't want to use a technique like this all the time - you end up with loads of symlinks in your bin directory, but in the right situation it can be very useful.


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